The importance of bio-fuels for transport has become more and more important in the past years and this trend is expected to continue. The main reasons are following i) depletion of crude oil reserves and significant increases in crude oil prices ii) increased environmental awareness particularly relating to emission of greenhouse gases iii) energy supply security.
Replacement of fossil fuels for automotive transport is one of the most critical areas closely related to reduction of greenhouse gas emissions. Among the fuels used for internal combustion engines, more than 700 million tons of diesel fuel was consumed in 2006. Diesel fuel demand is predicted to grow to 900 million tons by 2020. The share of diesel range fuels based on renewable feedstock's (biodiesel, gas-to-liquid (GTL) and green diesel) was only about 2% in 2006.
New legislation passed during 2007 calls for substantial increase in the proportion of renewable automotive fuels by 2020. Novel processes for conversion of biomass to automotive fuels are developed worldwide, however technological break-throughs are needed to reach the desired economics and life cycle efficiency goals. In the context of ways to reduce the use of fossil fuels, one approach has gained considerable attention and involves conversion of various vegetable oils to automotive fuels. Typical strategies in materializing this approach involve (i) transesterification route to obtain fuel composition often referred to as Biodiesel and (ii) hydrogenation route to obtain fuel composition often referred to as renewable diesel.
At present transesterification route has been demonstrated on commercial scale throughout the world. Within transesterification route, the vegetable oils with typical examples being palm oil, soybean oil and canola oil are reacted with methanol or other low-molecular weight alkyl alcohol to form fatty acid methyl ester (FAME) type Biodiesel fuel. The Biodiesel fuel obtained via transesterification route often referred to as “first generation biofuel”, however, suffers from several drawbacks including (i) competition for raw materials typically utilized in food applications, (ii) deforestation in tropical countries and (iii) issues related to diesel engine performance.
While the first two drawbacks concern serious moral issues, the later drawback, namely issues related to the engine performance, has proven to be a major obstacle from practical point of view in establishment of Biodiesel as fossil diesel alternative. Typical problems when Biodiesel fuels of FAME type have been used in diesel engine include: (i) deposits throughout the fuel and combustion systems, (ii) problems related to the poor cold-flow properties of FAME, (iii) low stability towards heat and oxidation, (iv) prone towards water absorption, (v) higher emissions of NOx relative to fossil type diesel and (vi) shortening of engine oils lifetime, etc. These problems can be eliminated if the vegetable oils or FAME type biofuels are treated at oil processing plants under hydrogen rich conditions (hydrogenation) alone or in combination with mineral oil fractions. Using the hydrogenation route, paraffin, iso-paraffin and cyclo-alkane rich fuels with excellent properties for direct use in modern diesel engines can potentially be produced. For example, the National Resources Canada (NRCan) has developed a process for the catalytic hydrogenation of vegetable oils over conventional NiMo/Al2O3 and CoMo/Al2O3 catalysts. Although, the biogas oil products of this process purportedly have cetane numbers in the range of 70-90, the yield of desired diesel range hydrocarbons is lower than about 60%. The by-products obtained during this catalytic hydrogenation are carbon oxides, short chain hydrocarbons, gasoline range hydrocarbons and very heavy high boiling hydrocarbons.
The notation “diesel range feedstock/fuel” in here is to be understood hydrocarbons and their oxygenated derivatives boiling in the range 170-400 degrees C.
While edible vegetable oils are used in food applications and are available in limited quantities only, other non-edible raw materials” would be suitable feedstock for the production of diesel range hydrocarbons. Processing of such raw materials is more extensive and involves fundamental changes in the properties within the comprising components and hence these raw materials and corresponding fuel products are often referred to as “second generation raw materials for biofuels and second generation biofuels”, respectively. A suitable example for such raw material comprising C14-C24 oxygenated hydrocarbons is crude tall oil. Crude tall oil is a viscous yellow to dark brown odorous liquid obtained as a by-product of Kraft process within wood pulp manufacture. The name originated as Anglicization of Swedish word “tallolja” (“pine oil”).
Crude tall oil, in the following CTO, contains unsaponifiable fraction (10-40 wt. %) and acidic fraction. The acidic fraction can be further sub-divided into diterpenic (rosin) carboxy-acids with general formula C20H30O2 (mainly Abietic acid and its isomers) and fatty acids ranging C14-C24 (principle components being Linoleic acid, Oleic acid, Linolenic acid and Palmitic acid i.e. C18 and C16). The unsaponifiable fraction comprises of fatty alcohols (C20-C24), some sterols (C30), and various other alkyl- and cyclo-alkyl hydrocarbon derivates (C10-C30). The acidic fraction of crude tall oil along with certain components comprising the unsaponifiable fraction is of particular interest in view of conversion to diesel fuel for direct use in engines or as high-quality feedstock suitable for the production of diesel range fuel compositions. Nowadays fractional distillation of CTO produces tall oil rosin and tall oil fatty acids (TOFA). The rosin finds use as a component of adhesives, rubbers, inks, and emulsifiers, whereas tall oil fatty acids find use in the production of soaps and lubricants.
Crude tall oil, however, as being by-product stream contains a long list of contaminants. Typical CTO contaminants that need to be mentioned include alkali salts, alkaline earth metal salts, solubilised iron, sulfur and sulfur-containing organic compounds, cellulosic fibers and large organic lignin compounds with molecular weights well over 1000 units.
There is direct relation between the amount of entrained black liquor material in CTO and the CTO contaminants and in particular between black liquor content and levels of lignin, fibers/foreign matter, sodium and calcium salts as well as calcium resinates and fatty acid calcium soaps. The acidulation of tall oil soap (tall oil precursor) with sulfuric acid at pulp mills does not entirely convert calcium resinates and fatty acid calcium soaps into their corresponding acid forms and calcium sulphate. Therefore, significant quantities of calcium remain in the CTO, in the order of several hundred ppm up to several thousand ppm. A large portion of the ash in CTO is calcium bound in soaps/resinates. Another major contaminant within CTO is lignin of varying fragment lengths. Lignin is a large and complex phenolic polymer that to a certain extent dissolves in the crude tall oil in the form phenolic fragments of varying molecular weight. The lignin content within CTO may vary between few hundred ppm in well acidulated and subsequently separated CTO to up to 10 000 ppm (or higher) for poorly acidulated and/or separated crude tall oils. Fibers are fine cellulose in nature entities not recovered within the wood pulp manufacturing. Typical fiber levels in CTO are 0.3-0.5 percent by weight. Yet another typical CTO contaminant is iron. Most often iron quantities are leached from non-protected surfaces of tanks, pipes, etc. which have been in contact with crude tall oil, especially at prolonged exposure. The leaching is largely promoted by the sulfuric acid entrained within the crude tall oil. The common feature of all these contaminants is that they have detrimental effect on the various catalyst utilized at oil refineries. In view of the CTO potential as second generation diesel fuel or as second generation feedstock for biofuel compositions these contaminants should preferably be removed from the CTO prior any direct use or catalytic upgrading.
The CTO contains various amounts of sulfur compounds ranging from about 500 ppm up to several thousand ppm. The sulfur compounds include a wide range of organic and inorganic sulfur compounds including sulphate, sulfite, polysulfide, elemental sulfur, mercaptans, organic sulfides and organic sulfones and sulfonates. Whereas some sulfur functionalities (—SH, —S—S—, —SOx, where x index can be 2, 3 and 4) are connected to both fatty and diterpenic moieties comprising the crude tall oil, most of the sulfur is concentrated into the CTO volatile fraction and to lower extend into the high boiling point fraction of CTO.
Crude tall oil has been proposed as a source of raw material suitable for the production of components boiling in the diesel range when processed via hydrogenation route in an existing hydroprocessing plants. However, the presence of various contaminants described above within CTO, have led to fast catalyst deactivation and undesired low yield of diesel range hydrocarbons. It is recognized that in particular sodium, calcium, fibers and lignin contaminants of CTO will shorten the life length of hydrogenation catalysts. Among these, catalyst coking, plugging and catalyst sites poisoning are major consequences caused by the presence of CTO contaminants.
Depitched tall oil, traditionally produced by a single evaporation stage performed on crude tall oil in a thin film evaporator, has been proposed as a feasible feedstock alternative for the production of diesel range fuels via a hydrogenation route. Traditional depitching is performed at high temperature (above 250 degrees C.) in one evaporative step resulting in that a large portion of the diesel range molecules are lost with the pitch. Furthermore high boiling compounds are entrained into the product distillate. Therefore more efficient separation procedures are needed if the product should be used as a diesel fuel or used as a feed to hydrogenation plants for production of high-quality diesel range products in yields above about 80%.
Clearly, the crude tall oil contains fractions of undesirable compounds that need to be either removed or converted before it can be used efficiently as a fuel or as a feed to hydroprocessing plants for production of diesel range fuel compositions.
We have discovered that crude tall oil can be converted into a high-quality diesel fuel or renewable feedstock for hydroprocessing plants (in the following Second Generation Crude Tall Diesel (SGCTD)) in very high yield suitable for use in hydrotreating-type processing units, wherein the SGCTD feedstock is treated with hydrogen to form low sulfur content diesel range fuel compositions. Optionally as described in the method of present invention, the SGCTD feedstock can be depleted of oxygen by decarboxylation/decarboxylation prior to or during a hydrotreating step.